1. Field of the Invention
This invention relates generally to networks that receive and transmit groups of logic signals from remote locations and, more particularly, to the verification of operation of the thermocouple elements located at remote sites in a control network.
2. Description of the Related Art
In the automatic control of processes, process control networks are provided to collect status information by at least one central location and to distribute control signals to the devices coupled to the network. Referring to FIG. 1, a portion of such a process control network is shown. The process control network includes a system bus 5 which has at least one central control unit 6 coupled thereto and at least one bus interface module 9 also coupled thereto. The central control unit 6 receives signals from the system bus 5 that typically communicate the status of user devices coupled to the process control network and applies signals to the network that provide the control signals determining the operation of the user devices. The bus interface module 9 applies status signals to the system bus and receives signals from the system bus 5 which are relevant to a subset of network user devices 10 that communicate with the central control unit 6 through bus interface module 9. User devices 10 capable of responding to digital signals, such as motors, lamps and heaters, receive control signals through device interface unit 16 and digital output module 12 from the bus interface unit 9. User devices 10 providing digital status signals in response to contact closures provide status signals to the bus interface module 9 by means of device interface unit 17 and digital input module 13. User devices 10 such as control valves, responding to analog control signals, receive signals from the bus interface module 9 through output analog module 14 and device interface unit 18. User devices 10 such as flowmeters, level detectors, pressure meters, temperature sensors, etc. of the type providing analog status information, provide the status signals to bus interface module 9 through device interface unit 19 and analog input module 15. The device interface units 16, 17, 18 and 19 convert the signals received from and transmitted to the user devices 10 into a format usable by the digital input and output modules and the analog input and output modules, respectively. The digital input and output modules 13 and 12 and the analog input and output modules 15 and 14 couple the signals of the user devices, as transformed by the device interface unit, with the bus interface module 9 by means of local bus 11.
The status information applied to the process control network has an accuracy limited by the accuracy of the sensor elements providing signals to a central control unit determined by environmental conditions. One of the important environmental factors in many processes is the temperature. The temperature is typically determined by a thermocouple element. Because of the critical nature of the temperature in the control of processes, failure of a thermocouple can compromise large amounts of processing activity prior to the identification of a failure. In the related art, a thermocouple has been tested by placing a voltage source and a large resistor in series with a thermocouple and checking for continuity. Using this technique, only catastrophic failures were detected and a thermocouple could provide erroneous reading prior to detectable failure.
A need has therefore been felt for apparatus and method that can operate continuously to identify the failure of a thermocouple to prevent the distribution of erroneous data.